Transitive and intransitive verbs
Transitive verbs take a - sometimes omitted - direct object, intransitive verbs do not.
Example (transitive) 买 (mǎi):
我买了一本书。
Wǒ mǎi le yī běn shū.
'I bought a book.'
Example (intransitive) 笑 (xiào):
他们都笑了。
Tāmen dōu xiào le.
'They all laughed.'
Some verbs can be used transitivly and intransitivly like 认识(rènshi). The meaning is linked but not identical.
Example:
你认识他的朋友吗?
Nǐ rènshi tā de péngyou ma?
'Do you know his friend?'
我们认识一下。
Wǒmen rènshi yī xià.
'Let's get acquainted.'